1. Field of the Invention
The present invention relates to an improved method of synthesizing cubic boron nitride from hexagonal boron nitride.
2. Description of the Related Art
Cubic boron nitride is second only to diamond in hardness but has a greater chemical stability and, therefore it is becoming increasingly more important as a grinding, polishing and cutting material. A variety of methods have been proposed for producing cubic boron nitride, but the most well-known of these, which is widely used industrially, is a method of converting hexagonal boron nitride to cubic boron nitride under high-temperature, high-pressure conditions of about 4.5-6.0 GPa and about 1400-1600.degree. C. in the presence of a solvent (catalyst). The well-known solvents (catalysts) for this method have conventionally been nitrides and boronitrides of alkali metals and alkaline earth metals. (see, for example, U.S. Pat. No. 3,772,428).
Nevertheless, the cubic boron nitrides obtained by use of said solvents (catalysts) have problems of low toughness and heat resistance, resulting in breakage of abrasives or of significant decrease in the strength of abrasives when exposed to high temperature. Also, the cubic boron nitrides obtained by use of said solvents (catalysts) have irregular shapes or nearly spherical shapes and are poor in the development of euhedral planes.